1. Field of the invention
The present invention relates to an improvement in compaction devices and methods for concrete block molding machines.
The invention is more particularly concerned with the making of concrete blocks requiring better and/or more uniform or selected physical properties, such as compressive strength, durability, flexural strength, permeability, density, color uniformity, etc. and intended for use with regard to any related physical properties including heavy and repeated pounding, as well as under severe weather exposure, such as pavement stones. Other like products contemplated by the invention, are solid or hollow construction blocks or bricks, cubes, cylinders, edging stones, concrete rings or pipes, curb stones, impermeable or permeable elements or any elements made from a no-slump or close to no slump concrete mix, having any particular water/cement ratio, in order to obtain better uniformity and any particularities of the physical properties of concrete. In the present specification, these elements are generically called concrete blocks, for convenience.
2. Description of the prior art
Present day skilled manufacturers use a machine which, broadly speaking, has a feed zone where a hopper pours a no-slump concrete mix of any chosen water/cement ratio into a drawer which is thereafter moved laterally and over the vibrating cells of a mold standing in an adjacent compaction zone. After the cells are filled with the mix, and initially compacted a predetermined lapse of feed time, the drawer is returned to the feed zone, being provided with suitable means to scrape off the tops of the cells, as it moves back to the feed zone. Final compaction, while vibration is going on, is achieved thereafter during a predetermined finishing time, by means of a plurality of fixed compaction shoes, provided at the lower ends of a like number of solid rod or tubing of fixed lengths, of which the upper ends are also fixedly mounted on a vertically movable head plate. For final compaction, the head plate is moved toward the mold, or vise versa, and a given pressure is then applied to the head plate, and hence, to the mix in each cell, by the fixed compaction shoes with or without vibration applied to the head plate and of course transmitted to the compaction shoes, that are solid with the head plate, as aforesaid. After a predetermined period of time of vibration in this last position, finished concrete blocks, are moved out of the mold onto a steel plate or on a steel pallet.
A major difficulty with this type of equipment is that, because the compaction shoes are located at the ends of solid rods or tubings of equal length, and have their flat working surfaces constantly at the same level, that is in the same horizontal plane, final compaction pressure is unequally applied in the various cells, even with vibration on the head. Indeed, the cells are never equally filled and compacted when the drawer backs up, even so, that the top surfaces of the mix in all the cells are at the same level. Thereafter, when comes the final compaction with the compaction shoes, the result is that the concrete mix is fully compacted in some of the cells, and less in others during the period of final vibration and therefore the density in the resulting blocks is not equal. It follows that, the less dense blocks do not have the expected or required physical properties capable of meeting the stiff specifications of CAN3-A231,2 or A.S.T.M.C936-82 for concrete paving stones, or other concrete blocks for which utmost density, and hence particularities of physical properties, is a most important requirement. In addition of not making full use of the cement powder, these less compacted or dense elements are prone to fail prematurely, or do not have the same properties anticipated. For these reasons, the quality of the finished products is variable and some may have to be rejected. Likewise, other concrete blocks which are less compacted by present day methods, fail to meet their anticipated physical properties and specifications.
Extensive research has demonstrated that all physical properties of a concrete block resulting from a no-slump mix, are very closely related to the density of the compacted mix and this holds true at all water/cement ratios within the no-slump range. This includes the lowest W/C ratio where a minimum of water is necessary to complete the hydration of the cement. But, the same does not apply so significantly to a slump, or near slump, concrete mix where maximum or near maximum density is more easily obtained in each cell due to the hence better workability of the mix, and, where also the physical properties are more uniform but where compressive strength or any other physical properties are often inadequate for different type of concrete blocks or intended to be exposed to severe conditions. It can also be verified that a higher W/C ratio mix may give a higher compressive strength than a lower one due to its better workability and greater density, but such is not the case of its physical properties.
Abram's law which states that the compressive strength of concrete is inversely proportional to the W/C ratio in the mix, applies to a no-slump concrete at any W/C ratio of the no-slump range but it is verified that the Abram's law holds true only if the proper maximum density is reached for all mixes in that ratio range. The compressive strength, flexural strength, durability and permeability are therefore functions of the density at any particular W/C ratio in the no-slump range and any drop of density from the utmost one appreciably reduces the compressive strength and other physical properties. As well, further drop in density causes the undesired water absorption capacity of the concrete to increase and may render the ensuing blocks less weather resistant. Conclusively, the compressive strength and related physical properties are directly proportional to the density at any W/C ratio.
Another interesting fact, noted with lower W/C ratio no-slump concrete, is the linear percentage of the loss of weight of the initial dry mass found between 25 and 50 cycles when the mass is subjected to freezing and thawing, durability tests conducted under CAN3-A231.2M85 specifications for paving stones of ultimate density. There again, the density of the finished concrete blocks is of prime importance and losses in weight in percent of the dry mass increase radically as densities decrease. This is most important and, according to the present invention, a minimum acceptable density can be obtained or found for each mix design and W/C ratio, which will permit a loss of weight of less than 1% under the above specifications and, at the same time, provide durability of all concrete blocks when, as aforesaid, made in accordance with the present invention.
There thus presently exists a problem of uneven properties in all concrete products when made according to the present day methods, due to the fact that these concrete products do not have the appropriate minimum acceptable density for their corresponding W/C ratio mix as above explained, with resulting uneven lower compressive strength, permeability, flexural strength, durability and related properties at least regarding an important number of concrete blocks so made.
In attempting to alleviate the problem of getting full compaction with existing machine, the industry is trying to work at higher W/C ratio to obtain a better density and compressive strength but this is achieved only at great expense in waste of cement and beneficial properties of lower W/C ratio mix. Thus, some devices like heated shoes have been designed. They permit the use of a higher W/C ratio mix and consequently a better workability of the mix but to prevent material from sticking to the shoes, heat has to be used. Of course, a higher workability obtained by adding water does give a higher density but unfortunately, when considering Abram's law, a higher W/C ratio gives lower compressive, flexural and shear strengths, higher water absorption, higher percentage loss of weight, in durability test due to freezing and thawing, and higher shrinkage.
Also, because the mix in some of the mold cells, in present day machines, are still not sufficiently compacted, some other devices, like vibration devices on the head and/or on the foot of the machine, have been used in an attempt to optimize the density. However, the same problem of inacceptable uneven compaction of some of the elements from different cells and consequently too high a disparity in physical properties of end products persist. Some cities using much de-icing salts are now considering not to use certain concrete blocks like paving stones, curbs, etc. in their streets because of their poor durability. The same problem applies to other prefabricated concrete blocks like bricks or pipes where any anticipated physical property such as permeability is not attained because of the uneven density at any W/C ratio mix.